Lithium battery

ABSTRACT

Provided herein is a lithium battery including: a cathode including a cathode active material; an anode including an anode active material; an electrolyte between the cathode and the anode; and a separator impregnated with the electrolyte, wherein the separator includes cellulose nanofibers, and wherein a differential scanning calorimetry (DSC) thermogram of the separator evinces an exothermic reaction peak, represented by a differential value (dH/dT), at a temperature in a range of about 150° C. to about 200° C.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No.10-2018-0088152, filed on Jul. 27, 2018, in the Korean IntellectualProperty Office, the entire disclosure of which is hereby incorporatedherein by reference.

BACKGROUND 1. Field

The present disclosure relates to lithium batteries.

2. Description of the Related Art

Electrochemical batteries such as lithium secondary batteries include aseparator to separate a positive electrode from a negative electrode,thereby preventing a short circuit. Separators must be tolerant to anelectrolyte solution and must exhibit low internal resistance. Recently,demand for heat-resistant electrochemical batteries for use inautomobiles has increased. A porous film formed of a polyolefin-basedmaterial such as polyethylene or polypropylene is sometimes used as aseparator in lithium secondary batteries. However, since batteries forautomobiles require resistance to high temperatures, i.e., 150° C. ormore, it is difficult to apply polyolefin-based separators to thesebatteries.

Cellulose-containing porous films have high heat resistance, and thusare suitable for use as separators of batteries for automobiles. Beforebeing used, separators including cellulose are impregnated withelectrolytes for lithium batteries. This can result in a side reactionbetween a cellulose-containing separator and an electrolyte, whichdeteriorates the performance of lithium batteries. Accordingly, there isa need for a lithium battery including a cellulose-containing separatorin which such a side reaction is suppressed.

SUMMARY

Provided are lithium batteries in which a side reaction of a separatorwith an electrolyte is suppressed at high temperatures. According to anaspect of the disclosure, a lithium battery includes: a cathodeincluding a cathode active material; an anode including an anode activematerial; an electrolyte between the cathode and the anode; and aseparator impregnated with the electrolyte, wherein the separatorincludes cellulose nanofibers, and an exothermic reaction peakrepresented by a differential value (dH/dT) of a thermogram is free at atemperature in a range of about 150° C. to about 200° C. at the time ofdifferential scanning calorimetry (DSC) measurement of the separatorimpregnated with the electrolyte.

Additional aspects will be set forth in part in the description whichfollows and, in part, will be apparent from the description, or may belearned by practice of the presented embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readilyappreciated from the following description of the embodiments, taken inconjunction with the accompanying drawings in which:

FIG. 1 is a differential scanning calorimetry (DSC) thermogram ofseparators prepared according to Examples 1 and 2 and ComparativeExample 1;

FIG. 2A is an image of a separator not impregnated with an electrolytesolution after heat exposure;

FIG. 2B is an image of a separator impregnated with a mixed solventafter heat exposure;

FIG. 2C is an image of a separator impregnated with the electrolytesolution used in Example 2 after heat exposure;

FIG. 2D is an image of a separator impregnated with the electrolytesolution used in Example 1 after heat exposure;

FIG. 2E is an image of a separator impregnated with the electrolytesolution used in Comparative Example 1 after heat exposure;

FIG. 2F is an image of a separator impregnated with the electrolytesolution used in Comparative Example 2 after heat exposure;

FIGS. 3A to 3H are schematic cross-sectional views of porous filmsaccording to example embodiments; and

FIG. 4 is a schematic view of a lithium battery according to anembodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings, wherein like referencenumerals refer to like elements throughout. In this regard, the presentembodiments may have different forms and should not be construed asbeing limited to the descriptions set forth herein. Accordingly, theembodiments are merely described below, by referring to the figures, toexplain aspects. As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items. Expressionssuch as “at least one of,” when preceding a list of elements, modify theentire list of elements and do not modify the individual elements of thelist.

As the present disclosure allows for various changes and numerousembodiments, example embodiments will be illustrated in the drawings anddescribed in the detailed description in detail. However, this is notintended to limit the present disclosure to particular modes ofpractice, and it is to be appreciated that all changes, equivalents, andsubstitutes that do not depart from the spirit and technical scope ofthe present disclosure are encompassed in the present disclosure.

The terminology used in the following description is used only todescribe specific embodiments and is not intended to limit the presentdisclosure. An expression in the singular includes an expression in theplural unless the content clearly indicates otherwise. In the followingdescription, it should be understood that terms, such as “include” and“have”, are used to indicate the presence of stated features, numbers,steps, operations, elements, parts, components, materials, or acombination thereof described in the specification without excluding inadvance the possibility of the presence or addition of one or more otherfeatures, numbers, steps, operations, elements, parts, components,materials, or combinations thereof.

In the drawings, thicknesses are enlarged or reduced to clearlyrepresent various layers and regions. Throughout the specification, likereference numerals denote like elements. In the entire specification,when a portion of a layer, a film, a region, a plate, or the like isreferred to as being “on” or “above” another portion, it includes notonly a case in which the portion is directly on the other portion, butalso a case in which an intervening portion is present therebetween.Throughout the specification, although terms such as “first,” “second,”and the like may be used to described various components, suchcomponents must not be limited by the above terms. The above terms areused only to distinguish one component from another.

Hereinafter, lithium batteries according to example embodiments will bedescribed in more detail.

A lithium battery according to an embodiment includes: a cathodeincluding a cathode active material; an anode including an anode activematerial; an electrolyte disposed between the cathode and the anode; anda separator impregnated with the electrolyte, wherein the separatorincludes cellulose nanofibers. Impregnated, as used herein consistentwith its plain meaning and ordinary usage, means that the separatorcomprises the electrolyte within the material of the separator, i.e.,between the fibers. In other words, the separator is partially orcompletely saturated with the electrolyte, as would occur form soakingthe separator in the electrolyte. Thus, for instance, the separator maybe disposed in the electrolyte contained in the battery. Furthermore,the separator is such that a differential scanning calorimetry (DSC)measurement of the separator impregnated with the electrolyte is free of(i.e., does not show) an exothermic reaction peak represented by adifferential value (dH/dT) of a thermogramat a temperature ranging fromabout 150° C. to about 200° C.

When an exothermic reaction peak represented by a differential value(dH/dT) of a thermogram is free at a temperature ranging from about 150°C. to about 200° C. at the time of differential scanning calorimetry(DSC) measurement of the separator impregnated with the electrolyte, aside reaction of the separator and the electrolyte is suppressed, andthus deterioration due to the side reaction of the separator and theelectrolyte is suppressed in the lithium battery including the separatorand the electrolyte. Referring to the thermogram of Comparative Example1 of FIG. 1, there is a region where the slope of the line increases andthen decreases within the temperature range of about 150° C. to about200° C. This increase and decrease in slope is a peak when representedby differential values. In contrast, referring to the thermograms ofExamples 1 and 2 also presented in FIG. 1, the slope of the line issubstantially over the same temperature range of about 150° C. to about200° C. Thus, no peak is shown when represented by a differential value.For example, the “exothermic reaction peak” is a peak represented by adifferential value at a certain region of DSC thermogram, wherein thecertain region of DSC thermogram sequentially has a first low sloperegion (region A), a high slope region (region B), and a second lowslope region (region C), and an angel between a first tangent line atthe first low slope region (region A) and a second tangent line at thehigh slope region (region B) is at least 30°.

In a lithium battery according to certain embodiments, after theseparator impregnated with the electrolyte is exposed to heat at 170° C.for 3 hours under an inert atmosphere, the crystalline index of theseparator after the heat exposure is 50% or more, 55% or more, 60% ormore, 65% or more, 70% or more, 75% or more, 80% or more, 85% or more,90% or more, or 95% or more the crystalline index of the separatorbefore the heat exposure. For example, when the crystalline index of theseparator before the heat exposure is 60, the crystalline index of theseparator after the heat exposure is 30 or more. The crystalline indexof the separator, as measured by an X-ray diffraction (XRD) spectrum, isexpressed in terms of an intensity ratio ((I₀₀₂−I_(AM))/I₀₀₂) of acrystalline peak intensity (I₀₀₂−I_(AM)) to a total peak intensity I₀₀₂of a (002) crystal plane, wherein the crystalline peak intensity is adifference between the total peak intensity I₀₀₂ of the (002) crystalplane and a peak intensity I_(AM) of an amorphous phase. Even when theseparator impregnated with the electrolyte is exposed to heat for a longperiod of time, the side reaction of the electrolyte and the separatoris suppressed, and thus the deterioration of crystallinity of theseparator is suppressed. For example, the separator impregnated with theelectrolyte is prepared by completely submerging the separator in theelectrolyte for 30 minutes at room temperature.

In the lithium battery according to certain embodiments, after theseparator impregnated with the electrolyte is exposed to heat at 170° C.for 3 hours under an inert atmosphere, carbonized areas are notsubstantially present on the surface of the separator. For example, nocarbonized area (e.g., black or brown discoloration) is visible with thenaked eye on the separator surface.

In the lithium battery, for example, the electrolyte includes at leastone selected from an ionic metal complex represented by Formula 1 and alithium salt containing a sulfonyl group (—S(═O)₂—):

In Formula 1, Me is an element selected from transition metals andelements belonging to groups 13 to 15 of Periodic Table; M is a metalion; a is an integer of 1 to 3, b is an integer of 1 to 3, and s=b/a; pis 0 to 8, q is 0 or 1, and r is 1 to 4; X₁ and X₂ are independently O,S, or NR_(a); R₁ and R_(a) are independently halogen, a C₁-C₅ alkylgroup unsubstituted or substituted with one or more halogens, or a C₁-C₅aryl group unsubstituted or substituted with one or more halogens; andR₂ is a C₁-C₅ alkylene group unsubstituted or substituted with ahalogen, or a C₁-C₅ arylene group unsubstituted or substituted with oneor more halogens.

For example, in the lithium battery, the ionic metal complex can berepresented by Formula 2:

In Formula 2, Me is Al, B, or P; M is a metal ion; p is 0 to 8, q is 0or 1, and r is 1 to 4; X₃ and X₄ are independently O or S; R₃ is ahalogen; and R₄ is a C₁-C₅ alkylene group unsubstituted or substitutedwith one or more halogens, or a C₁-C₅ arylene group unsubstituted orsubstituted with one or more halogens.

For example, in the lithium battery, the ionic metal complex can berepresented by Formulae 3 or 4:

In Formulae 3 and 4, Me is Al, B, or P; p is 0 to 8, and r is 1 to 4; R₅is halogen; and R₆ is a C₁-C₅ alkylene group unsubstituted orsubstituted with one or more halogens.

For example, in the lithium battery, the ionic metal complex can berepresented by one of Formulae 5 to 10.

In the lithium battery, for example, the lithium salt containing thesulfonyl group is selected from LiCF₃SO₃, LiC₄F₉SO₃, Li(CF₃SO₂)₂N,Li(C₂F₅SO₂)₂N, Li(FSO₂)₂N, Li(CF₃SO₃)₂N, andLiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1) SO₂), wherein x and y are eachindependently an integer of 3 to 10.

In the lithium battery, the content of at least one selected from theionic metal complex and the lithium salt containing the sulfonyl groupincluded in the electrolyte is about 0.01M to about 5M, about 0.1M toabout 4M, about 0.5M to about 3M, or about 1M to about 2M based on thetotal weight of the electrolyte. When the content of the ionic metalcomplex and the lithium salt containing the sulfonyl group is within theabove range, the side reaction of the electrolyte and the separator isfurther suppressed.

In some embodiments of the lithium battery, the electrolyte may notinclude (may be free of) a sulfonyl group-free lithium salt, such asLiPF₆ or LiBF₄.

According to certain embodiments, the electrolyte is, for example, anorganic electrolyte including an organic solvent. The organicelectrolyte may be prepared by adding the ionic metal complex and thelithium salt containing the sulfonyl group to an organic solvent.Examples of the organic solvent included in the organic electrolyteinclude, but are not limited to, propylene carbonate, ethylenecarbonate, fluoroethylene carbonate, butylene carbonate, dimethylcarbonate, diethyl carbonate, methyl ethyl carbonate, methyl propylcarbonate, ethyl propyl carbonate, methyl isopropyl carbonate, dipropylcarbonate, dibutyl carbonate, benzonitrile, acetonitrile,tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, dioxolane,4-methyldioxolane, N,N-dimethylformamide, dimethylacetamide,dimethylsulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane,dichloroethane, chlorobenzene, nitrobenzene, diethylene glycol, anddimethyl ether. Any organic solvent known in the art may be used. Theelectrolyte may not include an ionic liquid.

Alternatively, according to certain embodiments, the electrolyte is asolid electrolyte. Examples of the solid electrolyte include, but arenot limited to, boron oxide and lithium oxyntride. Any solid electrolyteknown in the art may be used. The solid electrolyte is formed on ananode or a cathode by sputtering, atomic layer deposition (ALD),chemical vapor deposition (CVD), physical vapor deposition (PVD), or thelike. Alternatively, the electrolyte is a gel electrolyte.

The cellulose nanofibers included in the separator comprise, forexample, natural cellulose such as plant cellulose nanofibers, animalcellulose nanofibers, or microbial cellulose nanofibers. Non-limitingexamples of the cellulose nanofibers include coniferous wood pulp,deciduous wood pulp, cotton-based pulp such as cotton linter,non-wood-based pulp such as wheat straw pulp and bagasse pulp, celluloseseparated from bacterial cellulose or Ascidiacea, and celluloseseparated from seaweed. The cellulose nanofibers are, for example,microbial cellulose nanofibers.

The cellulose nanofibers included in the separator are, for example,carboxyl group-containing cellulose nanofibers. For example, thecarboxyl group of the cellulose nanofibers of the separator is acarboxyl group bound to carbon atoms forming a pyranose ring, and thecarboxyl group is represented by Formula a or b below:

—R₁—O—R₂—COOM  <Formula a>

—O—R₂—COOM.  <Formula b>

In Formulae a and b, R₁ and R₂ are each independently a substituted orunsubstituted alkylene group of 1 to 10 carbon atoms, and M is hydrogenor an alkali metal. For example, the alkali metal is lithium, sodium,potassium, or the like. For example, R₁ and R₂ are each independently amethylene group. For example, the carboxyl group bound to carbon atomsforming a pyranose ring, included in the carboxyl group-containingcellulose nanofibers, is —CH₂OCH₂COONa or —OCH₂COONa. The pyranose ringis, for example, glucopyranose. Thus, the carboxyl group of Formula a orb included in the carboxyl group-containing cellulose nanofibers has aspecific structure different from that of a carboxyl group having theformula of —COOM which is bound to carbon atoms forming a pyranose ringin conventional oxidized cellulose nanofibers obtained through chemicaloxidation.

The content of the carboxyl group of the carboxyl group-containingcellulose nanofibers used as the cellulose nanofibers included in theseparator is, for example, 0.02 mmol/g or more, 0.06 mmol/g or more,0.10 mmol/g or more, 0.15 mmol/g or more, or 0.20 mmol/g or more. Thecontent of the carboxyl group of the carboxyl group-containing cellulosenanofibers included in the separator is, for example, about 0.02 mmol/gto about 10 mmol/g, about 0.02 mmol/g to about 5 mmol/g, about 0.02mmol/g to about 3 mmol/g, about 0.02 mmol/g to about 2 mmol/g, or about0.02 mmol/g to about 1 mmol/g. Since the cellulose nanofibers includethe carboxyl group-containing cellulose nanofibers having a carboxylgroup content within the above ranges, the separator including the sameprovides further enhanced tensile strength and tensile modulus. For amethod of measuring the content of the carboxyl group of the cellulosenanofibers, refer to Evaluation Example 6.

As used herein, the “average diameter” of cellulose nanofibers is avalue obtained by capturing transmission electron microscope (TEM)images of a plurality of fibers, obtaining the diameters of 100 fibersusing an image analyzer for the TEM images, and then averaging thesediameters.

The carboxyl group-containing cellulose nanofibers included in theseparator may have an average diameter of, for example, 100 nm or less,80 nm or less, 60 nm or less, 50 nm or less, 45 nm or less, 40 nm orless, 35 nm or less, 30 nm or less, or 25 nm or less. The averagediameter of the cellulose nanofibers included in the separator is, forexample, about 1 nm to about 300 nm, about 1 nm to about 200 nm, about 1nm to about 100 nm, about 1 nm to about 50 nm, about 1 nm to about 45nm, about 1 nm to about 40 nm, about 1 nm to about 35 nm, or about 1 nmto about 25 nm. Since the separator includes the carboxylgroup-containing cellulose nanofibers having an average diameter withinthe above ranges, the tensile strength of the separator is furtherenhanced. For a method of measuring the average diameter of the carboxylgroup-containing cellulose nanofibers, refer to Evaluation Example 7.

In a diameter distribution curve of the carboxyl group-containingcellulose nanofibers included in the separator, a full width at halfmaximum (FWHM) of a diameter peak may be, for example, 100 nm or less,50 nm or less, 45 nm or less, 40 nm or less, 35 nm or less, 30 nm orless, 25 nm or less, 20 nm or less, 15 nm or less, or 10 nm or less. Inthe diameter distribution curve of the carboxyl group-containingcellulose nanofibers, the FWHM of the diameter peak may be, for example,about 1 nm to about 95 nm, about 1 nm to about 45 nm, about 5 nm toabout 45 nm, or about 10 nm to about 45 nm. The separator including thecarboxyl group-containing cellulose nanofibers having a FWHM within theabove narrow ranges exhibits further enhanced uniformity, and furtherenhanced tensile strength due to increased contacts between fibers.

The carboxyl group-containing cellulose nanofibers may be, for example,carboxyl group-containing microbial or bacterial cellulose nanofibers.That is, the carboxyl group-containing microbial or bacterial cellulosenanofibers are, for example, a fermentation product of a culturesolution including a microorganism and directly obtained from a culturesolution including a microorganism. Thus, the carboxyl group-containingmicrobial or bacterial cellulose nanofibers included in the separatorare distinguished from a simple mixture of existing general microbial orbacterial cellulose nanofibers and a carboxyl group-containing compound.In addition, the carboxyl group-including microbial or bacterialcellulose nanofibers are also distinguished from wood-based cellulosenanofibers obtained by decomposing a wood-based material. The carboxylgroup-containing microbial or bacterial cellulose nanofibers used as thesecond cellulose nanofibers have an absorption peak corresponding to acarboxyl group at around 1,572 cm⁻¹ in an infrared (IR) spectrum.Carboxyl group-free microbial or bacterial cellulose does not have suchan absorption peak.

The carboxyl group-containing microbial or bacterial cellulosenanofibers are obtained using, for example, a microorganism derived fromthe genus Enterobacter, the genus Gluconacetobacter, the genusKomagataeibacter, the genus Acetobacter, the genus Achromobacter, thegenus Agrobacterium, the genus Alcaligenes, the genus Azotobacter, thegenus Pseudomonas, the genus Rhizobium, the genus Sarcina, the genusKlebsiella, or the genus Escherichia, but the present disclosure is notnecessarily limited to the above examples, and the carboxylgroup-containing microbial or bacterial cellulose nanofibers may be anymicroorganism that produces microbial or bacterial cellulose in the art.The microorganism derived from the genus Acetobacter may be, forexample, Actetobacter pasteurianus. The microorganism derived from thegenus Agrobacterium may be, for example, Agrobacterium tumefaciens. Themicroorganism derived from the genus Rhizobium may be, for example,Rhizobium leguminosarum. The microorganism derived from the genusSarcina may be, for example, Sarcina ventriculi. The microorganismderived from the genus Gluconacetobacter may be, for example,Gluconacetobacter xylinum. The microorganism derived from the genusKlebsiella may be, for example, Klebsiella pneumoniae. The microorganismderived from the genus Escherichia may be, for example, E. coli.

In certain embodiments, the separator further includes, as the cellulosenanofibers, a combination of different types of cellulose nanofibers inaddition to the microbial or bacterial cellulose nanofibers. Forexample, the separator further includes wood-based cellulose nanofibers,but the present disclosure is not necessarily limited thereto. Anysuitable cellulose nanofibers known in the art that are capable ofenhancing the tensile strength of a separator may be used.

The thickness of the separator is, for example, 10 μm or more. Thethickness of the separator is, for example, 10 μm to 500 μm, 10 μm to450 μm, 10 μm to 400 μm, 10 μm to 350 μm, 10 μm to 300 μm, 10 μm to 250μm, 10 μm to 200 μm, 10 μm to 150 μm, 10 μm to 100 μm, 10 μm to 80 μm,10 μm to 60 μm, 10 μm to 40 μm, or 10 μm to 30 μm. The thickness of theseparator is, for example, 10 μm to 30 μm. When the thickness of theseparator is too thin, the mechanical properties of the separatordeteriorates, and such a separator may be damaged at the time ofassembling a lithium battery employing the separator, or may be damagedby lithium dendrite generated during charging/discharging of the lithiumbattery, and thus the lithium battery deteriorates. When the separatoris too thick, the internal resistance of the lithium battery employingthe separator increases and impairs the cycle characteristics of thelithium battery. Use of a separator that is excessively thick results inan increase in volume of the lithium battery, thus lowering the energydensity of the lithium battery per unit volume. Therefore, as thethickness of the separator decreases, the thickness of the lithiumbattery including the separator decreases, so that the energy density ofthe lithium battery per unit volume increases.

The air permeability of the separator according to certain embodimentsmay be expressed as a Gurley value, of, for example, about 50 sec/100 ccto about 800 sec/100 cc, about 100 sec/100 cc to about 750 sec/100 cc,about 150 sec/100 cc to about 700 sec/100 cc, about 200 sec/100 cc toabout 650 sec/100 cc, about 250 sec/100 cc to about 600 sec/100 cc,about 300 sec/100 cc to about 600 sec/100 cc, about 350 sec/100 cc toabout 600 sec/100 cc, about 350 sec/100 cc to about 550 sec/100 cc, orabout 350 sec/100 cc to about 500 sec/100 cc. The Gurley value ismeasured using a method in accordance with Japanese Industrial Standards(JIS) P8117. When the Gurley value is too low, lithium is easilydeposited in pores of the separator. Thus, when a separator having avery low Gurley value is used for a lithium battery, lithium blockingcharacteristics deteriorate, and thus a short circuit due to lithiumdendrites easily occurs. When the Gurley value of the porous film is toohigh, the transfer of lithium ions through the separator is inhibited.Thus, when a separator having a very high Gurley value is used for alithium battery, internal resistance of the lithium battery isincreased, resulting in deteriorated cycle characteristics of thelithium battery. In addition, the separator has a uniform Gurley valueover its entire area. Since the separator has uniform air permeability,current density is uniformly distributed in an electrolyte of a lithiumbattery including the porous film as a separator, and thus a sidereaction, such as deposition of crystals at an interface between anelectrode and an electrolyte, is suppressed.

The separator according to certain embodiments may have a porosity of,for example, about 10% to about 90%, about 15% to about 85%, about 20%to about 80%, about 25% to about 80%, about 30% to about 80%, about 35%to about 80%, about 35% to about 75%, or about 40% to about 75%. Evenwhen the porosity of the separator is less than 10%, a lithium batterymay still operate, but internal resistance thereof is increased, andthus output is reduced, resulting in deteriorated performance of thelithium battery. When the porosity of the separator is greater than 90%,internal resistance is excessively reduced, which may result in enhancedoutput characteristics of a lithium battery, for example, cyclecharacteristics of a lithium battery, but increases the likelihood of ashort circuit due to lithium dendrites, resulting in reduced stability.The porosity of the separator is measured using a liquid or gasadsorption method in accordance with ASTM D-2873 (Standard Test Methodfor Interior Porosity of Poly(Vinyl Chloride) (PBC) Resins by MercuryIntrusion Porosimetry).

The separator may have a tensile strength of, for example, 200 kgf/cm²or more, 250 kgf/cm² or more, 300 kgf/cm² or more, 350 kgf/cm² or more,360 kgf/cm² or more, 380 kgf/cm² or more, 400 kgf/cm² or more, 420kgf/cm² or more, 440 kgf/cm² or more, 460 kgf/cm² or more, 460 kgf/cm²or more, 480 kgf/cm² or more, 500 kgf/cm² or more, 550 kgf/cm² or more,600 kgf/cm² or more, 650 kgf/cm² or more, 700 kgf/cm² or more, 750kgf/cm² or more, 800 kgf/cm² or more, 850 kgf/cm² or more, 900 kgf/cm²or more, or 950 kgf/cm² or more. The tensile strength of the separatormay be, for example, about 200 kgf/cm² to about 1,000 kgf/cm², about 300kgf/cm² to about 1000 kgf/cm², about 50 kgf/cm² to about 800 kgf/cm²,about 50 kgf/cm² to about 700 kgf/cm², about 400 kgf/cm² to about 1000kgf/cm², about 500 kgf/cm² to about 1000 kgf/cm², about 500 kgf/cm² toabout 950 kgf/cm², about 500 kgf/cm² to about 900 kgf/cm², about 500kgf/cm² to about 850 kgf/cm², about 500 kgf/cm² to about 800 kgf/cm²,about 500 kgf/cm² to about 750 kgf/cm², or about 500 kgf/cm² to about700 kgf/cm². When the tensile strength of the separator is within theabove range, it is possible to achieve a minimum tensile strengthrequired for the manufacture of a winding-type battery, and pin-puncturestrength is further enhanced. Thus, when such a separator is employed,the durability of the separator increases during charging anddischarging of a lithium battery, and the thickness of the separator isreduced, and, accordingly, battery capacity is further increased. Whenthe tensile strength of the separator is less than 200 kgf/cm², thedurability of the separator decreases, which leads to breakage of theseparator when a battery is manufactured, resulting in reducedmanufacturing yield, and it is impossible to manufacture a winding-typebattery. In addition, when the tensile strength of the porous film isless than 50 kgf/cm², pin-puncture strength is low and thus durabilityis low, and the thickness of the separator, required to secure minimumtension, is increased, resulting in deteriorated battery capacity. Thetensile strength of the separator is measured in accordance withJapanese Industrial Standards (JIS) K 7127.

The separator according to certain embodiments may have a pin-puncturestrength (in terms of kilogram-force kgf) of, for example, 30 kgf ormore, 50 kgf or more, 70 kgf or more, 75 kgf or more, 80 kgf or more, 85kgf or more, 90 kgf or more, 95 kgf or more, or 100 kgf or more. Thepin-puncture strength of the separator may be, for example, about 30 kgfto about 150 kgf, about 50 kgf to about 150 kgf, about 70 kgf to about150 kgf, about 75 kgf to about 150 kgf, about 80 kgf to about 150 kgf,about 80 kgf to about 145 kgf, about 85 kgf to about 140 kgf, about 90kgf to about 140 kgf, or about 90 kgf to about 130 kgf. When thepin-puncture strength of the separator is within the above range, it ispossible to effectively suppress a short circuit due to dendrite duringcharging and discharging. Thus, when such a separator is employed, thedurability of the separator increases during charging and discharging ofa lithium battery, deterioration of the battery is suppressed, thethickness of the separator is reduced, and the battery capacity furtherincreases. When the penetration strength of the separator is less than30 kgf, the durability of the separator decreases, and thus thethickness of the separator, required to secure minimum pin-puncturestrength, is increased, resulting in deteriorated battery capacity. Themethod of measuring the pin-puncture strength of the separator is notparticularly limited, and any method known in the art may be used.

In at least one embodiment, in addition to the cellulose nanofibers, theseparator further includes at least one selected from a cross-linkingagent, a binder, inorganic particles, and polyolefin. These componentsmay be included or excluded in order to adjust physical properties ofthe separator. If the separator further includes a cross-linking agentand/or a binder, the tensile strength of the separator can be furtherenhanced.

The cross-linking agent aids in binding of cellulose nanofibers. Thecross-linking agent may be added in an amount of about 1 part by weightto about 50 parts by weight with respect to 100 parts by weight ofcellulose nanofibers, but the amount of the cross-linking agent is notnecessarily limited to the above range, and may be within any range thatmay enhance physical properties of the separator. The amount of thecross-linking agent ranges from, for example, about 1 part by weight toabout 30 parts by weight, about 1 part by weight to about 20 parts byweight, or about 1 part by weight to about 15 parts by weight, withrespect to 100 parts by weight of cellulose nanofibers. Thecross-linking agent may be, for example, at least one selected fromisocyanate, polyvinyl alcohol, and polyamide epichlorohydrin (PAE), butis not necessarily limited to the above examples, and may be anycross-linking agent that may be used in the art.

The binder aids in binding of cellulose nanofibers. The binder may beadded in an amount of about 1 part by weight to about 50 parts by weightwith respect to 100 parts by weight of cellulose nanofibers, but theamount of the binder is not necessarily limited to the above range, andmay be within any range that may enhance physical properties of theseparator. The amount of the binder ranges from, for example, about 1part by weight to about 30 parts by weight, about 1 part by weight toabout 20 parts by weight, or about 1 part by weight to about 15 parts byweight, with respect to 100 parts by weight of cellulose nanofibers. Thebinder may be, for example, at least one selected from cellulose singlenanofiber, methyl cellulose, hydroxypropyl methylcellulose, hydroxyethylmethyl cellulose, carboxyl methyl cellulose, ethyl cellulose,polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidenefluoride-co-trichloroethylene, polymethylmethacrylate,polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate,polyethylene-co-vinyl acetate, polyimide, polyethylene oxide, celluloseacetate, cellulose acetate butyrate, cellulose acetate propionate,cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose,cyanoethylsucrose, pullulan, and polyvinylalcohol, but the presentdisclosure is not necessarily limited to the above examples, and thebinder may be any binder used in the art.

The inorganic particles enhance mechanical properties of the separator.The inorganic particles may be added in an amount of about 0.01 parts byweight to about 20 parts by weight with respect to 100 parts by weightof cellulose nanofibers, but the amount of the inorganic particles isnot necessarily limited to the above range, and may be within any rangethat may enhance physical properties of the separator. The amount of thebinder ranges from, for example, about 1 part by weight to about 30parts by weight, about 1 part by weight to about 20 parts by weight, orabout 1 part by weight to about 15 parts by weight, with respect to 100parts by weight of cellulose nanofibers. Examples of the inorganicparticles include: a metal oxide selected from alumina (Al₂O₃), silica(SiO₂), titania (TiO₂), HfO₂, SrTiO₃, SnO₂, CeO₂, Na₂O, MgO, NiO, CaO,BaO, ZnO, ZrO₂, Y₂O₃, BaTiO₃, Li₂O, RaO, CaO, SrO, Sc₂O₃, Ce₂O₃, andcage-type silsesquioxane; a metal nitride selected from ZrN, TaN, HfN,VN, NbN, Cr₂N, TaN, CrN, GeN, TLi₃N, Mg₃N₂, Ca₃N₂, Sr₃N₂, Ba₃N₂, BN,AlN, and TiN; a metal oxynitride selected from tantalum oxynitride(TaON), zirconium oxynitride (ZrO_(x)N_(y) where 0<x<2, 0<y<3), andlithium phosphorus oxynitride (LiPON); a metal carbide selected fromTiC, ZrC, HfC, NbC, TaC, Cr₃C₂, Mo₂C, WC, and SiC; a metal-organicframework (MOF); a lithiated compound of each of the above-listedcompounds; one or more ceramic conductors selected fromLi_(1+x+y)Al_(x)Ti_(2−x)Si_(y)P_(3−y)O₁₂ where 0<x<2 and 0≤y<3, BaTiO₃,Pb(Zr,Ti)O₃ (PZT), Pb_(1−x)La_(x)Zr_(1−y)Ti_(y)O₃ (PLZT) where 0≤x<1 and0≤y<1, Pb(Mg₃Nb_(2/3))O₃—PbTiO₃ (PMN-PT), lithium phosphate (Li₃PO₄),lithium titanium phosphate (Li_(x)Ti_(y)(PO₄)₃ where 0<x<2 and 0<y<3,lithium aluminum titanium phosphate (Li_(x)Al_(y)Ti_(z)(PO₄)₃ where0<x<2, 0<y<1, and 0<z<3, Li_(1+x+y)(Al, Ga)x(Ti,Ge)_(2−x)Si_(y)P_(3−y)O₁₂ where 0≤x≤1 and 0≤y≤1, lithium lanthanumtitanate (Li_(x)La_(y)TiO₃ where 0<x<2 and 0<y<3), lithium germaniumthiophosphate (Li_(x)Ge_(y)P_(z)S_(w) where 0<x<4, 0<y<1, 0<z<1, and0<w<5), lithium nitride (Li_(x)N_(y) where 0<x<4 and 0<y<2),SiS₂(Li_(x)Si_(y)S_(z) where 0<x<3, 0<y<2, and 0<z<4)-based glass,P₂S₅(Li_(x)P_(y)S_(z) where 0<x<3, 0<y<3, and 0<z<7)-based glass,Li₂O—Al₂O₃—SiO₂—P₂O₅—TiO₂—GeO₂-based ceramic, and Garnet-based ceramicLi_(3+x)La₃M₂O₁₂ where 0≤x≤5, wherein M=Te, Nb, or Zr; and acarbonaceous nanostructure such as graphene, carbon nanotubes (CNTs),and carbon nanofibers (CNFs), but the present disclosure is not limitedto the above examples, and the inorganic particles may be any inorganicparticles used in the art. The inorganic particles may have a diameterof, for example, about 1 nm to about 10 μm, about 10 nm to about 6 μm,or about 100 nm to about 1 μm. The inorganic particles are disposed, forexample, inside the separator and/or on one surface of the separator.

As used herein, the “size” indicates an average particle diameter whenparticles are spherical. The “size” indicates a major axis length whenparticles are rod-shaped or elliptical. As used herein, the “averageparticle size” or “average particle diameter” refers to a particlediameter (D50) corresponding to 50% of particle diameters in adistribution curve where particles with the smallest particle diameterand particles with the largest particle diameter are accumulated inorder. Here, the total number of accumulated particles is 100%. Theaverage particle size may be measured according to methods known tothose skilled in the art. For example, the average particle size may bemeasured using a particle size analyzer, TEM images, or SEM images. Asanother method of measuring the average particle size, there is a methodusing a measuring device using dynamic light scattering. According tothis method, the number of particles having a predetermined size rangemay be counted, from which the average particle size may be calculated.

Polyolefin, which is a material of a general separator or the separatorof the present disclosure, enhances the flexibility of the separator.Examples of the polyolefin include polyethylene and polypropylene. Thepolyolefin is formed into a porous film that is either a single-layerfilm or a multi-layer film including at least two layers. For example,the polyolefin is formed into a two-layer film ofpolyethylene/polypropylene, a three-layer film ofpolyethylene/polypropylene/polyethylene, a three-layer film ofpolypropylene/polyethylene/polypropylene, or the like, but the presentdisclosure is not limited to the above examples, and the polyolefin maybe any polyolefin that may be used in the art.

In some embodiments, the porous film has a low contact angle withrespect to a polar solvent, such as water, and thus provides enhancedwettability with respect to an electrolyte in a polar solvent. A contactangle of one exemplary separator with respect to water at 20° C. may be,for example, 60° or less, 50° or less, 40° or less, 30° or less, or 20°or less. When the contact angle of the separator with respect to waterat 20° C. is too high, it is difficult to impregnate the porous filmwith an electrolyte. When the separator is used as a separator of alithium battery, the separator provides enhanced wettability withrespect to an electrolyte, and thus an interface between the separatorand an electrode is uniformly impregnated with the electrolyte. Thus, auniform electrode reaction proceeds at the interface between theseparator and an electrode, and, accordingly, the formation of lithiumdendrites (for example, due to a local overcurrent, or the like), isprevented, resulting in enhanced lifespan characteristics of a lithiumbattery.

The porous film has excellent thermal stability at a high temperature(for example, temperatures of 150° C. or higher), thus improving thethermal resistance of an electrochemical cell including the porous filmas a separator. In certain embodiments, the porous film may have a heatshrinkage rate, after being maintained at 150° C. for 30 minutes, of 5%or less, 4.5% or less, 4% or less, 3.5% or less, 3% or less, 2.5% orless, 2% or less, 1.5% or less, or 1% or less. In contrast, currentlyavailable olefin-based porous films rapidly contract at a hightemperature, i.e., between 150° C. and 200° C., and thus operation of abattery including such a porous film is stopped.

An exemplary separator according to an embodiment may have a heatshrinkage rate, after being maintained at 150° C. for 30 minutes, of 5%or less, 4.5% or less, 4% or less, 3.5% or less, 3% or less, 2.5% orless, 2% or less, 1.5% or less, or 1% or less. Since the separatorprovides excellent thermal stability at a high temperature, i.e., at150° C. or more, a lithium battery including the separator exhibitsenhanced heat resistance. In contrast, currently available generalolefin-based separators rapidly contract at a high temperature, i.e.,between 150° C. and 200° C., and thus operation of a lithium battery isstopped.

The separator has various single-layered structures or multi-layeredstructures according to required performance. Separators according toexample embodiments having a single-layered or multi-layered structurewill be described with reference to FIGS. 3A to 3H.

Referring to FIG. 3A, a separator 4 has a single-layered structureincluding a first layer 4 a including cellulose nanofibers. The firstlayer 4 a may include, for example, microbial cellulose nanofibershaving an average particle diameter of 50 nm or less as the cellulosenanofibers.

Referring to FIG. 3B, the separator 4 has a multi-layered structureincluding: a first layer 4 a including cellulose nanofibers; and asecond layer 4 b disposed on one surface of the first layer 4 a andincluding a polyolefin. The first layer 4 a may include, for example,microbial cellulose nanofibers having an average particle diameter of 50nm or less as the cellulose nanofibers. The second layer 4 b mayinclude, for example, at least one polyolefin selected from polyethyleneand polypropylene.

Referring to FIG. 3C, the separator 4 has a multi-layered structureincluding: a first layer 4 a including cellulose nanofibers; a secondlayer 4 b disposed on one surface of the first layer 4 a and including apolyolefin; and a third layer 4 b disposed on the other surface of thefirst layer 4 a and including a polyolefin. The first layer 4 a mayinclude, for example, microbial cellulose nanofibers having an averageparticle diameter of 50 nm or less as the cellulose nanofibers. Thesecond layer 4 b and the third layer 4 b may include, for example, atleast one polyolefin selected from polyethylene and polypropylene. Thesecond layer 4 b and the third layer 4 b, for example, may have the samecomposition.

Referring to FIG. 3D, the separator 4 has a multi-layered structureincluding: a first layer 4 a including cellulose nanofibers; and asecond layer 4 c disposed on one surface of the first layer 4 a,including cellulose nanofibers, and having a composition different fromthat of the first layer 4 a. The first layer 4 a may include, forexample, microbial cellulose nanofibers having an average particlediameter of 50 nm or less as the cellulose nanofibers. The second layer4 c may include, for example, microbial cellulose nanofibers having anaverage particle diameter of 50 nm or less and wood-based cellulosenanofibers having an average particle diameter of 100 nm or more as thecellulose nanofibers.

Referring to FIG. 3E, the separator 4 has a multi-layered structureincluding: a first layer 4 a including cellulose nanofibers; a secondlayer 4 c disposed on one surface of the first layer 4 a, includingcellulose nanofibers, and having a composition different from that ofthe first layer 4 a; and a third layer 4 c disposed on the other surfaceof the first layer 4 a, including cellulose nanofibers, and having acomposition different from that of the first layer 4 a. The first layer4 a may include, for example, microbial cellulose nanofibers having anaverage particle diameter of 50 nm or less as the cellulose nanofibers.The second layer 4 c and the third layer 4 c may include, for example,microbial cellulose nanofibers having an average particle diameter of 50nm or less and wood-based cellulose nanofibers having an averageparticle diameter of 100 nm or more as the cellulose nanofibers. Thesecond layer 4 c and the third layer 4 c, for example, may have the samecomposition.

Referring to FIG. 3F, the separator 4 has a multi-layered structureincluding: a first layer 4 a including cellulose nanofibers; a secondlayer 4 b disposed on one surface of the first layer 4 a and including apolyolefin; and a third layer 4 c disposed on the other surface of thefirst layer 4 a, including cellulose nanofibers, and having acomposition different from that of the first layer 4 a. The first layer4 a may include, for example, microbial cellulose nanofibers having anaverage particle diameter of 50 nm or less as the cellulose nanofibers.The second layer 4 b may include, for example, at least one polyolefinselected from polyethylene and polypropylene. The third layer 4 c mayinclude, for example, microbial cellulose nanofibers having an averageparticle diameter of 50 nm or less and wood-based cellulose nanofibershaving an average particle diameter of 100 nm or more as the cellulosenanofibers.

Referring to FIG. 3G, the separator 4 has a multi-layered structureincluding: a first layer 4 b including a polyolefin; a second layer 4 adisposed on one surface of the first layer 4 b and including cellulosenanofibers; and a third layer 4 a disposed on the other surface of thefirst layer 4 b and including cellulose nanofibers. The second layer 4 aand the third layer may include, for example, microbial cellulosenanofibers having an average particle diameter of 50 nm or less as thecellulose nanofibers. The second layer 4 b may include, for example, atleast one polyolefin selected from polyethylene and polypropylene. Thesecond layer 4 a and the third layer 4 a, for example, may have the samecomposition.

Referring to FIG. 3H, the separator 4 has a multi-layered structureincluding: a second layer 4 b including a polyolefin; a first layer 4 adisposed on one surface of the second layer 4 b and including cellulosenanofibers; and a third layer 4 c disposed on the other surface of thesecond layer 4 b, including cellulose nanofibers, and having acomposition different from that of the first layer 4 a. The first layer4 a may include, for example, microbial cellulose nanofibers having anaverage particle diameter of 50 nm or less as the cellulose nanofibers.The second layer 4 b may include, for example, at least one polyolefinselected from polyethylene and polypropylene. The third layer 4 c mayinclude, for example, microbial cellulose nanofibers having an averageparticle diameter of 50 nm or less and wood-based cellulose nanofibershaving an average particle diameter of 100 nm or more as the cellulosenanofibers.

Referring to FIGS. 3A to 3H, at least one of the first layer 4 a, thesecond layer 4 b, and the third layer 4 c, included in the separator 4,may further include, or may not include at least one selected from across-linking agent, a binder, inorganic particles, and a polyolefin.

In the lithium battery according to certain embodiments, the cathodeactive material includes a lithium transition metal oxide includingnickel and another transition metal. The content of nickel in thelithium transition metal oxide including nickel and another transitionmetal is 60 mol % or more, 65 mol % or more, 70 mol % or more, 75 mol %or more, 80 mol % or more, 85 mol % or more, 87 mol % or more, or 90 mol% or more with respect to the total number of moles of transitionmetals.

For example, the lithium transition metal oxide is represented byFormula 11:

Li_(a)Ni_(x)CO_(y)M_(z)O_(2−b)A_(b).  <Formula 11>

In Formula 11, 1.0≤a≤1.2, 0≤b≤0.2, 0.6≤x<1, 0<y≤0.3, 0<z≤0.3, andx+y+z=1 are satisfied; M is at least one selected from manganese (Mn),vanadium (V), magnesium (Mg), gallium (Ga), silicon (Si), tungsten (W),molybdenum (Mo), iron (Fe), chromium (Cr) Zinc (Zn), titanium (Ti),aluminum (Al), and boron (B); and A is F, S, Cl, Br, or a combinationthereof. For example, 0.65≤x<1, 0<y≤0.34, 0<z≤0.34; 0.7≤x<1, 0<y≤0.29,0<z≤0.29; 0.75≤x<1, 0<y≤0.24, 0<z≤0.24; 0.8≤x<1, 0<y≤0.19, 0<z≤0.19;0.82≤x<0.97, 0<y≤0.15, 0<z≤0.15; 0.85≤x<0.95, 0<y≤0.15, and 0<z≤0.15 maybe satisfied.

For example, the lithium transition metal oxide is represented byFormulae 12 or 13:

LiNi_(x)CO_(y)Mn_(z)O₂  <Formula 12>

LiNi_(x)CO_(y)Al_(z)O₂.  <Formula 13>

In Formulae 12 and 13, 0.8≤x≤0.95, 0<y≤0.19, and 0<z≤0.19 are satisfied.For example, 0.82≤x≤0.95, 0<y≤0.15, and 0<z≤0.15 are satisfied. Forexample, 0.85≤x≤0.95, 0<y≤0.15, and 0<z≤0.15 may be satisfied.

For example, the lithium transition metal oxide may beLiNi_(0.7)Co_(0.2)Mn_(0.1)O₂, LiNi_(0.88)Co_(0.08)Mn_(0.04)O₂,LiNi_(0.8)Co_(0.15)Mn_(0.05)O₂, LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂,LiNi_(0.88)Co_(0.1)Mn_(0.02)O₂, LiNi_(0.8)Co_(0.15)Al_(0.05)O₂,LiNi_(0.8)Co_(0.1)Mn_(0.2)O₂, or LiNi_(0.88)Co_(0.1)Al_(0.02)O₂.

In a lithium battery according to certain embodiments, the anode activematerial includes at least one selected from a silicon-based compound, acarbon-based compound, a composite of a silicon-based compound and acarbon-based compound, and a silicon oxide (SiO_(x), 0<x<2). Thecarbon-based compound is, for example, graphite, but this is notparticularly limited and may be any suitable carbon-based compound knownin the art.

The composite of a silicon-based compound and a carbon-based compoundmay have a structure where silicon nanoparticles are disposed on acarbon-based compound, a composite where silicon nanoparticles areincluded on the surface of a carbon-based compound or inside thecarbon-based compound, or a composite where silicon nanoparticles arecoated with a carbon-based compound to included inside the carbon-basedcompound. The composite of a silicon-based compound and a carbon-basedcompound may be an active material obtained by dispersing siliconnanoparticles having an average particle diameter of about 200 nm orless on carbon-based compound particles and then performing carboncoating, or may be an active material where silicon (Si) nanoparticlesexist on and in graphite. The composite of a silicon-based compound anda carbon-based compound may include secondary particles having anaverage particle diameter of about 5 μm to about 20 μm. The averageparticle diameter of the silicon nanoparticles may be 5 nm or more, 10nm or more, 20 nm or more, 50 nm or more, or 70 nm or more. The averageparticle diameter of the silicon nanoparticles may be 200 nm or less,150 nm or less, 100 nm or less, 50 nm or less, 20 nm or less, or 10 nmor less. For example, the average particle diameter of the siliconnanoparticles may be about 100 nm to about 150 nm.

The average particle diameter of the secondary particles of thecomposite of a silicon-based compound and a carbon-based compound may beabout 5 μm to about 18 μm, about 7 μm to about 18 μm, about 7 μm toabout 15 μm, or about 10 μm to about 13 μm.

In the lithium battery according to certain embodiments, after thelithium battery is charged up to 100% of state of charge (SOC) and thenexposed to heat at 170° C. for 1 hour, a voltage of the lithium batteryafter the heat exposure is 90% or more, 93% or more, or 95% or more avoltage of the lithium battery before the heat exposure.

The lithium battery may be manufactured using, for example, thefollowing method, but the present disclosure is not limited thereto, andthe method may be any fabrication method known in the art that enablesthe operation of a lithium battery.

First, an anode is prepared according to an anode fabrication method.

An anode active material, a conductive material, a binder, and a solventmay be mixed to prepare an anode active material composition, and theanode active material composition may be directly coated on a currentcollector such as copper foil or the like to thereby fabricate an anodeplate. Alternately, the anode active material composition may be cast ona separate support and an anode active material film separated from thesupport may be laminated on a copper current collector to therebyfabricate an anode plate. The anode is not limited to theabove-described type, and may be of other types known in the art.

The anode active material in the lithium battery may be any anode activematerial known in the art. For example, the anode active material mayinclude at least one selected from lithium metal, a metal alloyable withlithium, a transition metal oxide, a non-transition metal oxide, and acarbonaceous material.

For example, the metal alloyable with lithium may be silicon (Si), tin(Sn), aluminum (Al), germanium (Ge), lead (Pb), bismuth (Bi), antimony(Sb), a Si-yttrium (Y) alloy (Y is an alkali metal, an alkali earthmetal, a Group 13 to 16 element, a transition metal, a rare earthelement, or a combination thereof except for Si), a Sn—Y alloy (Y is analkali metal, an alkali earth metal, a Group 13 to 16 element, atransition metal, a rare earth element, or a combination thereof exceptfor Sn), or the like. Examples of Y include magnesium (Mg), calcium(Ca), strontium (Sr), barium (Ba), radium (R_(a)), scandium (Sc),yttrium (Y), titanium (Ti), zirconium (Zr), hafnium (Hf), rutherfordium(Rf), vanadium (V), niobium (Nb), tantalum (Ta), dubnium (Db), chromium(Cr), molybdenum (Mo), tungsten (W), seaborgium (Sg), technetium (Tc),rhenium (Re), bohrium (Bh), iron (Fe), lead (Pb), ruthenium (Ru), osmium(Os), hassium (Hs), rhodium (Rh), iridium (Ir), palladium (Pd), platinum(Pt), copper (Cu), silver (Ag), gold (Au), zinc (Zn), cadmium (Cd),boron (B), aluminum (Al), gallium (Ga), tin (Sn), indium (In), titanium(Ti), germanium (Ge), phosphorus (P), arsenic (As), antimony (Sb),bismuth (Bi), sulfur (S), selenium (Se), tellurium (Te), polonium (Po),and any combination thereof.

The transition metal oxide may be, for example, lithium titanium oxide,vanadium oxide, lithium vanadium oxide, or the like.

The non-transition metal oxide may be, for example, SnO₂, SiO_(x) where0<x<2, or the like.

The carbonaceous material may be crystalline carbon, amorphous carbon,or a mixture thereof. Examples of the crystalline carbon include naturalgraphite and artificial graphite, each of which has an irregular form ora plate, flake, spherical, or fibrous form. Examples of the amorphouscarbon include, but are not limited to, soft carbon (low-temperaturecalcined carbon), hard carbon, mesophase pitch carbide, and calcinedcoke.

The anode active material includes at least one selected from asilicon-based compound, a carbon-based compound, a composite of asilicon-based compound and a carbon-based compound, and a silicon oxide(SiO_(x), 0<x<2). The conductive material may be acetylene black,natural graphite, artificial graphite, carbon black, Ketjen black,carbon fiber, metallic powder such as copper, nickel, aluminum, silver,or the like, metal fiber, or the like. In some embodiments, conductivematerials such as polyphenylene derivatives and the like may be usedalone or a mixture of two or more of these materials may be used, butthe present disclosure is not limited to the above-listed examples. Anyconductive material that is known in the art may be used. In addition,the above-described carbonaceous materials may also be used as aconductive material.

Examples of the binder in the anode include a vinylidenefluoride/hexafluoropropylene copolymer, polyvinylidene fluoride (PVDF),polyacrylonitrile, polymethyl methacrylate, polytetrafluoroethylene, amixture of the aforementioned polymers, and a styrene-butadienerubber-based polymer. However, the binder is not particularly limited tothe above examples and may be any binder that is known in the art.

The solvent in the anode may be N-methylpyrrolidone, acetone, water, orthe like. However, the solvent is not particularly limited to the aboveexamples and may be any solvent known in the art.

The amounts of the negative active material, the conductive material,the binder, and the solvent may be the same levels as those generallyused in a lithium battery. At least one of the conductive material andthe solvent may not be used according to the use and constitution ofdesired lithium batteries.

Next, a cathode is prepared according to a cathode fabrication method.

The cathode may be fabricated in the same manner as in anode fabricationmethod, except that a cathode active material is used instead of theanode active material. In addition, in a cathode active materialcomposition, a conductive material, a binder, and a solvent may be thesame as those used in the anode.

A cathode active material, a conductive material, a binder, and asolvent may be mixed to prepare a cathode active material composition,and the cathode active material composition may be directly coated on analuminum current collector to thereby fabricate a cathode plate.Alternately, the cathode active material composition may be cast onto aseparate support and a cathode active material film separated from thesupport may be laminated on an aluminum current collector to therebyfabricate a cathode plate. The cathode is not limited to theabove-described type, and may be of other types known in the art.

The cathode active material may include at least one selected fromlithium cobalt oxide, lithium nickel cobalt manganese oxide, lithiumnickel cobalt aluminum oxide, lithium iron phosphate, and lithiummanganese oxide. However, the positive active material is not limited tothe above examples and any cathode active material used in the art maybe used.

For example, the cathode active material may be a compound representedby one of the following formulae: Li_(a)A_(1−b)B_(b)D₂ where 0.90≤a≤1.8and 0≤b≤0; Li_(a)E_(1−b)B_(b)O_(2−c)D_(c) where 0.90≤a≤1.8, 0≤b≤0.5, and0≤c≤0.05; LiE_(2−b)B_(b)O_(4−c)D_(c) where 0≤b≤0.5 and 0≤c≤0.05;Li_(a)Ni_(1−b−c)Co_(b)B_(c)D_(α) where 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05,and 0<α≤2; Li_(a)Ni_(1−b−c)Co_(b)B_(c)O_(2−a)F_(α) where 0.90≤a≤1.8,0≤b≤0.5, 0≤c≤0.05, and 0<α<2; Li_(a)Ni_(1−b−c)Co_(b)B_(c)O_(2−α)F₂ where0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, and 0<α<2); Li_(a)Ni_(1−b−c)Mn_(b)B_(c)Dαwhere 0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, and 0<α≤2;Li_(a)Ni_(1−b−c)Mn_(b)B_(c)O_(2−α)F_(α) where 0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05, and 0<α<2; Li_(a)Ni_(1−b−c)Mn_(b)B_(c)O_(2−α)F₂ where0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, and 0<α<2; Li_(a)Ni_(b)E_(c)G_(d)O₂ where0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, and 0.001≤d≤0.1;Li_(a)Ni_(b)Co_(c)Mn_(d)G_(e)O₂ where 0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5,0≤d≤0.5, and 0.001≤e≤0.1; Li_(a)NiG_(b)O₂ where 0.90≤a≤1.8 and0.001≤b≤0.1; Li_(a)CoG_(b)O₂ where 0.90≤a≤1.8 and 0.001≤b≤0.1;Li_(a)MnG_(b)O₂ where 0.90≤a≤1.8 and 0.001≤b≤0.1; Li_(a)Mn₂G_(b)O₄ where0.90≤a≤1.8 and 0.001≤b≤0.1; QO₂; QS₂; LiQS₂; V₂O₅; LiV₂O₅; LiIO₂;LiNiVO₄; Li_((3−f))J₂(PO₄)₃ where 0≤f≤2; Li_((3−f))Fe₂(PO₄)₃ where0≤f≤2; and LiFePO₄.

In the formulae above, A may be selected from nickel (Ni), cobalt (Co),manganese (Mn), and a combination thereof; B may be selected fromaluminum (Al), Ni, Co, Mn, chromium (Cr), iron (Fe), magnesium (Mg),strontium (Sr), vanadium (V), a rare earth element, and a combinationthereof; D may be selected from oxygen (O), fluorine (F), sulfur (S),phosphorus (P), and a combination thereof; E may be selected from Co,Mn, and a combination thereof; F may be selected from F, S, P, and acombination thereof; G may be selected from Al, Cr, Mn, Fe, Mg,lanthanum (La), cerium (Ce), Sr, V, and a combination thereof; Q isselected from titanium (Ti), molybdenum (Mo), Mn, and a combinationthereof; I is selected from Cr, V, Fe, scandium (Sc), yttrium (Y), and acombination thereof; and J may be selected from V, Cr, Mn, Co, nickel(Ni), copper (Cu), and a combination thereof.

The cathode active material may be, for example, a composite furtherhaving a coating layer on a surface of the above-listed compound, or amixture of the above-listed compound and a compound having a coatinglayer. The coating layer may include a coating element compound, such asan oxide or hydroxide of a coating element, an oxyhydroxide of a coatingelement, an oxycarbonate of a coating element, or a hydroxycarbonate ofa coating element. These compounds constituting the coating layers maybe amorphous or crystalline. The coating element included in the coatinglayer may be, for example, magnesium (Mg), aluminum (Al), cobalt (Co),potassium (K), sodium (Na), calcium (Ca), silicon (Si), titanium (Ti),vanadium (V), tin (Sn), germanium (Ge), gallium (Ga), boron (B), arsenic(As), zirconium (Zr), or a mixture thereof. A coating layer may beformed using the aforementioned compounds and the coating elementsconstituting the compounds of the coating layers by using any one ofvarious coating methods that do not adversely affect physical propertiesof the cathode active material. The coating method may be, for example,spray coating, dipping, or the like. The coating method is wellunderstood by those of ordinary skill in the art, and thus, a detaileddescription thereof is omitted herein.

According to certain embodiments, the cathode active material is theaforementioned lithium transition metal oxide represented by formulae 1to 3 above.

Next, the aforementioned separator including cellulose nanofibers isdisposed between the cathode and the anode. An electrolyte including atleast one selected from an ionic metal complex compound represented bythe formula 1 and a lithium salt containing a sulfonyl group (—S(═O) 2-)is injected into the separator and thus the separator is impregnatedwith the injected electrolyte.

Referring to FIG. 4, a lithium battery 1 includes a cathode 3, an anode2, and a separator 4. The cathode 3, the anode 2, and the separator 4are wound or folded to be accommodated in a battery case 5.Subsequently, an organic electrolyte solution is injected into thebattery case 5 and the battery case 5 is sealed with a cap assembly 6 tothereby complete the manufacture of the lithium battery 1. The batterycase 5 may be of a cylindrical type, a rectangular type, a pouch type, acoin type, or the like. The lithium battery 1 may be, for example, athin-film type battery. The lithium battery 1 may be, for example, alithium ion battery.

The separator is disposed between the cathode and the anode to therebyform a battery assembly. A plurality of battery assemblies may bestacked in a bi-cell structure, and then impregnated with an organicelectrolyte solution, and the obtained resulting structure isaccommodated in a pouch and sealed, thereby completing the manufactureof a lithium-ion polymer battery.

A battery assembly or a plurality of battery assemblies form a batterymodule, and a plurality of battery modules are stacked to form a batterypack. The battery pack may be used in any device that requires highcapacity and high output. For example, the battery pack may be used in alaptop computer, a smart phone, an electric vehicle, or the like.

The lithium battery has excellent rate capability and excellent lifespancharacteristics, and is thus suitable for use in electric vehicles(EVs). For example, the lithium battery is suitable for use in hybridvehicles such as plug-in hybrid electric vehicles (PHEVs) and the like.

The separator according to certain embodiments is illustrativelyprepared by the following method, but the present disclosure is notnecessarily limited thereto. Any preparation method known in the artthat enables the preparation of a separator as a porous film may beused.

A method of preparing a separator includes: preparing a compositionincluding cellulose nanofibers, a hydrophilic pore-forming agent, and asolvent and applying the composition onto a substrate; drying thecomposition to form a sheet on the substrate; and separating the sheetfrom the substrate to obtain a separator consisting of the sheet.Inclusion of cellulose nanofibers, improves both the mechanicalproperties and air permeability of the separator, thus improving boththe energy density and power of a lithium battery including theseparator.

In the separator preparation method, water is used as a solvent, but thesolvent is not particularly limited. Any organic solvent known in theart, such as methanol, ethanol, propanol, butanol, or acetone may beused.

In the separator preparation method according to certain embodiments,the composition further includes at least one selected from across-linking agent and a binder. Inclusion of a cross-linking agentand/or a binder further enhances tensile strength of the separator. Thetypes of the cross-linking agent and the binder are the same as those ofthe cross-linking agent and the binder included in the aforementionedembodiment of a separator.

In certain embodiments, the hydrophilic pore-forming agent may be atleast one selected from: pore-forming agents in a liquid state at roomtemperature such as polyethylene glycol, ethylene carbonate, propylenecarbonate, vinylene carbonate, propanesulfone, ethylenesulfate,dimethylsulfone, ethyl methyl sulfone, dipropyl sulfone, dibutylsulfone, trimethylene sulfone, tetramethylene sulfone,di(methoxyethyl)sulfone (CH₃OCH₂CH₂)₂SO₂), and ethyl cyclopentyl sulfone(C₂H₅SO₂C₅H₉); and pore-forming agents in a liquid state at roomtemperature such as 1,5-pentanediol, 1-methylamino-2,3-propanediol,ε-caprolactone, γ-butyrolactone, α-acetyl-γ-butyrolactone, diethyleneglycol, 1,3-butylene glycol, propylene glycol, triethylene glycoldimethyl ether, tripropylene glycol dimethyl ether, diethylene glycolmonobutyl ether, triethylene glycol monomethyl ether, triethylene glycolbutyl methyl ether, tetraethylene glycol dimethyl ether, diethyleneglycol monoethyl ether acetate, diethylene glycol monoethyl ether,triethylene glycol monobutyl ether, tetraethylene glycol monobutylether, dipropylene glycol monomethyl ether, diethylene glycol monomethylether, diethylene glycol monoisopropyl ether, ethylene glycolmonoisobutyl ether, tripropylene glycol monomethyl ether, diethyleneglycol methyl ethyl ether, diethylene glycol diethyl ether, glycerin,propylene carbonate, and N-methylpyrrolidone. However, the presentdisclosure is not limited to the above examples, and any pore-formingagent known in the art may be used.

In a case in which the hydrophilic pore-forming agent in a solid stateis used at room temperature, the content of water decreases as the waterevaporates from the composition, such that an amount of the residualpore-forming agent exceeds the solubility of the pore-forming agent inwater; pore-forming agents in a solid state at room temperature areprecipitated from water and distributed and disposed within a sheet in asolid state; and thus, additional agglomeration or a rearrangement ofthe precipitated pore-forming agent, which is already distributed withinthe sheet, due to additional evaporation of water is suppressed. Thus,the pore size and pore distribution uniformity of the separator arefurther enhanced.

The method of preparing a separator further includes: washing the sheetor the separator with an organic solvent. By washing the sheet or theseparator with an organic solvent, the pore-forming agent remaining inthe sheet or the separator is effectively removed. The washing methodand the number of washing operations are not limited and the washingprocess may be performed once or more to adjust physical properties ofthe separator. The organic solvent used to wash the sheet or theseparator may be any organic solvent used in the art that is capable ofdissolving a hydrophilic pore-forming agent in a solid state at roomtemperature. For example, toluene is used as the organic solvent. Afterremoving the hydrophilic pore-forming agent using the organic solvent inthe separator preparation method, the temperature and time for dryingthe washed separator are not particularly limited, but the separator maybe dried, for example, at a temperature between about 20° C. and about120° C. for about 1 minute to about 10 hours. The drying process isperformed at atmospheric pressure or in a vacuum oven.

In the separator preparation method, the composition includes water as asolvent, but the present disclosure is not necessarily limited thereto.Alternately, the composition further includes, in addition to water,other solvents capable of dissolving cellulose nanofibers and ahydrophilic pore-forming agent in a solid state at room temperature.

In the separator preparation method, the amount of the cellulosenanofibers included in the composition is, for example, about 0.01 wt %to about 50 wt %, about 0.05 wt % to about 40 wt %, about 0.1 wt % toabout 30 wt %, about 0.2 wt % to about 20 wt %, about 0.3 wt % to about15 wt %, about 0.3 wt % to about 10 wt %, about 0.35 wt % to about 8 wt%, about 0.4 wt % to about 6 wt %, or about 0.4 wt % to about 5 wt %,with respect to the total weight of the composition. When the amount ofthe cellulose nanofibers is too small, an excessively large amount oftime is consumed in drying or the like, productivity is reduced, and thetensile strength of the separator is reduced. When the amount of thecellulose nanofibers is too large, the viscosity of the composition isexcessively increased, and thus it is difficult to obtain a uniformsheet.

In the separator preparation method, the drying temperature of water isnot particularly limited, but the drying process is performed at atemperature between about 50° C. and about 120° C. for about 1 minute toabout 10 hours. The drying process is performed at atmospheric pressureor in a vacuum oven.

Hereinafter, the present disclosure will be described in further detailwith reference to the following examples and comparative examples.However, these examples are provided for illustrative purposes only andare not intended to limit the scope of the present disclosure.

Preparation of Cellulose Nanofibers Preparation Example 1: Preparationof Carboxyl Group-Containing Microbial Cellulose Nanofibers

A wild-type Gluconacetobacter xylinum strain (KCCM 41431) was placed in700 ml of a Hestrin-Schramm (HS) medium supplemented with 1.0 w/v %carboxymethylcellulose (CMS) (Na-CMC, SIGMA) having a molecular weightof 250,000 Daltons in a 1 L fermenter (Hanil Science IndustryCorporation, GX LiFlus Series Jar-type open system, positive pressuremaintained to prevent contamination), and cultured at 30° C. for 48hours while being stirred at 200 rpm using an impeller. The HS mediumcontained 20 g/L of glucose, 5 g/L of bacto-peptone, 5 g/L of a yeastextract, 2.7 g/L of Na₂HPO₄, and 1.15 g/L of citric acid, in water.

A fermentation solution, in which carboxyl group-containing cellulosenanofibers produced as a result of culturing were uniformly dispersed,was collected. The collected fermentation solution was allowed to passtwice through a microchannel (Interaction chamber, size: 200 μm) of aNano Disperser (Ilshin Autoclave, ISA-NH500, Korea), which is ahigh-pressure homogenizer, in a state in which 70 MPa was appliedthereto, to thereby obtain a high-pressure homogenized fermentationsolution including carboxyl group-containing cellulose nanofibers. Thehigh-pressure homogenized fermentation solution including carboxylgroup-containing cellulose nanofibers was centrifuged to obtain acellulose precipitate. The obtained precipitate was heated at 90° C. ina 0.1N aqueous NaOH solution for 2 hours to hydrolyze cells andimpurities present between the carboxyl group-containing cellulosenanofibers, followed by rinsing with distilled water, thereby obtainingpurified carboxyl group-containing cellulose nanofibers.

The prepared carboxyl group-containing cellulose nanofibers had anaverage diameter of 18 nm, the content of carboxyl groups was 0.11mmol/g, and a weight average polymerization degree thereof was 5531 DPw.

Preparation of Separator Preparation Example 2

0.6 mg (2 wt % of total amount of dispersion) of propylene carbonate asa pore-forming agent was added to 30 ml of a 0.5 wt % (based onmicrobial cellulose nanofiers) dispersion in which a mixture of themicrobial cellulose nanofibers of Preparation Example 1 andcarboxymethylcellulose (CMC (Na-CMC, SIGMA)) in a weight ratio of 100:20was diluted with water, and the resulting solution was stirred at 1,000rpm and at room temperature for 1 hour. The obtained composition wasapplied onto a polyester film substrate to a thickness of 1.5 mm using amicrometer adjustable applicator, and dried at 85° C. for 3 hours toremove water and propylene carbonate to thereby obtain a porous film.The obtained porous film was not a woven film, but non-woven fabric. Theobtained porous film was used as a separator as it was.

Manufacture of Lithium Battery Example 1

(Manufacture of Cathode)

A cathode in which a mixture of LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ andLiNi_(0.8)Co_(0.15)Al_(0.05)O₂ in a weight ratio of 8:2 as a cathodeactive material, a carbon conductive material (Denka Black), andpolyvinylidene fluoride (PVdF) are mixed in a weight ratio of 96:1.8:2.2was used.

(Manufacture of Anode)

An anode in which graphite particles, a styrene-butadiene rubber (SBR)as a binder, and carboxymethylcellulose (CMC) are mixed in a weightratio of 98:1.2:0.8 was used.

(Manufacture of Lithium Battery)

The separator prepared according to Preparation Example 2 was used.

The porous film of Preparation Example 2 was interposed between thecathode and the anode, and then the resulting structure was accommodatedin a pouch and an electrolyte solution was injected thereinto, followedby sealing, thereby manufacturing a pouch cell.

An electrolyte solution in which an ionic metal complex represented byFormula 9 below is saturated in a mixed solvent of ethylene carbonate(EC):ethyl methyl carbonate (EMC):dimethyl carbonate (DMC) (volumeratio: 2:4:4) was used as the electrolyte solution.

Example 2

A pouch cell was manufactured in the same manner as in Example 1, exceptthat an electrolyte solution in which 1.15 M Li(CF₃SO₂)₂N is dissolvedinstead of the ionic metal complex represented by Formula 9 was used.

Comparative Example 1

A pouch cell was manufactured in the same manner as in Example 1, exceptthat an electrolyte solution in which 1.15 M LiPF₆ is dissolved insteadof the ionic metal complex represented by Formula 9 was used.

Comparative Example 2

A pouch cell was manufactured in the same manner as in Example 1, exceptthat an electrolyte solution in which 1.15 M LiBF₄ is dissolved insteadof the ionic metal complex represented by Formula 9 was used.

Evaluation Example 1: Measurement of Differential Scanning Calorimetry(DSC) of Separator

The separator prepared in Preparation Example 2 was impregnated witheach of the electrolyte solutions used in Example 1, Example 2, andComparative Example 1, and then a DSC thermogram was measured, and theresults thereof are shown in FIG.

As shown in FIG. 1, the separator impregnated with each of theelectrolytic solution used in Examples 1 and 2 had no significant changein the DSC thermogram up to 200° C. That is, it was found that theseparator impregnated with each of the electrolytic solution used inExamples 1 and 2 was not subjected to an exothermic reaction up to 200°C.

In contrast, in the separator impregnated with the electrolytic solutionused in Comparative Example 1, at 170° C., the slope of the DSCthermogram increased and then decreased again. That is, an exothermicreaction peak expressed by a differential value (dH/dT) corresponding tothe slope of the thermogram was obtained near 170° C. Therefore, it wasfound that the separator impregnated with the electrolytic solution usedin Comparative Example 1 was subjected to an exothermic reaction at 170°C.

Evaluation Example 2: Evaluation of Crystallinity of Separator

The XRD of the separator prepared in Preparation Example 2 was measured,and the crystalline index of the separator, which is an intensity ratio((I₀₀₂−I_(AM))/I₀₀₂) of a crystalline peak intensity (I₀₀₂−I_(AM)) to atotal peak intensity I₀₀₂ of a (002) crystal plane, was 0.64, whereinthe crystalline peak intensity is a difference between the total peakintensity I₀₀₂ of the (002) crystal plane and a peak intensity I_(AM) ofan amorphous phase. After the separator prepared in Preparation Example2 was impregnated with the electrolytic solution used in ComparativeExample 1, the separator was sealed, left near 170° C. for 3 hours,exposed to heat while blocking moisture in a dry room under an inert gasatmosphere, and then the XRD of the separator was measured to obtain acrystalline index of 0.26. The crystalline index of the separator afterheat exposure was 40.6% of the crystalline index of the separator beforeheat exposure.

Meanwhile, in the separator impregnated with the electrolyte solutionused in Example 2, the crystalline index of the separator after heatexposure was 97% of the crystalline index of the separator before heatexposure.

Evaluation Example 3: Evaluation of Carbonization of Separator

After the separator prepared in Preparation Example 2 was impregnatedwith each of the electrolyte solutions used in Examples 1 and 2 andComparative Examples 1 and 2 and a mixed solvent of EC (ethylenecarbonate):EMC (ethylmethyl carbonate):DMC (dimethyl carbonate) (volumeratio: 2:4:4), the separator was sealed, heated at about 170° C. for 3hours while blocking moisture in a dry room under an inert gasatmosphere, and then the carbonization of the separator was evaluatedwith the naked eye.

Separately, the separator prepared in Preparation Example 2 was notimpregnated with the electrolyte solution, and the carbonization of theseparator was evaluated under the same conditions.

FIG. 2A is an image of a separator not impregnated with an electrolytesolution after heat exposure.

FIG. 2B is an image of a separator impregnated with a mixed solventafter heat exposure.

FIG. 2C is an image of a separator impregnated with the electrolytesolution used in Example 2 after heat exposure.

FIG. 2D is an image of a separator impregnated with the electrolytesolution used in Example 1 after heat exposure.

FIG. 2E is an image of a separator impregnated with the electrolytesolution used in Comparative Example 1 after heat exposure.

FIG. 2F is an image of a separator impregnated with the electrolytesolution used in Comparative Example 2 after heat exposure.

As shown in FIGS. 2C to 2D, in the separators impregnated with theelectrolyte solutions of Examples 1 and 2, there was substantially nocarbonized area on the surface of the separator even after heatexposure. The surfaces of the separators impregnated with theelectrolyte solutions of Examples 1 and 2 are similar to the surfaces ofthe separator not impregnated with the electrolyte solution and theseparator impregnated with the mixed solvent.

In contrast, as shown in FIG. 2E, in the separator impregnated with theelectrolyte solution of Comparative Example 1, the surface of theseparator after heat exposure was entirely carbonized. Further, as shownin FIG. 2F, in the separator impregnated with the electrolyte solutionof Comparative Example 2, the surface of the separator after heatexposure was partially carbonized.

Therefore, in the lithium batteries of Examples 1 and 2, thecarbonization of the separator caused by a side reaction of theelectrolyte solution and the separator was prevented.

Evaluation Example 4: Evaluation of Charge/Discharge Characteristics

Each of the lithium batteries (pouch cells) manufactured according toExamples 1 and 2 and Comparative Example 1 was charged at a constantcurrent of 0.1 C rate at 25° C. until a voltage reached 4.2 V (vs. Li),and was then encapsulated again after removing a gas generated bydisassembling the pouch cell. After completing the gas removal, eachpouch cell was charged at a constant current of 0.2 C rate until avoltage reached 4.2 V (vs. Li), and was charged at a constant voltagemaintained at 4.3 V until a current reached 0.05 C. Each of thecompletely pouch cells was rested for 10 minutes, and was thendischarged at a constant current of 0.2 C until the voltage reached 2.8V (vs. Li) (1^(st) cycle, formation process).

Each pouch cell having gone through the formation process was charged ata constant current of 0.2 C rate until the voltage reached 4.3 V (vs.Li), and charged at a constant voltage maintained at 4.3 V until thecurrent reached 0.05 C. Each of the completely charged pouch cells wasrested for 10 minutes, and then each pouch cell was discharged at aconstant current of 0.2 C until the voltage reached 2.8 V (vs. Li). Thiscycle of charging and discharging was repeated two times (2^(nd)˜3^(rd)cycles).

Subsequently, each pouch cell was charged at a constant current of 0.2 Crate at 25° C. until the voltage reached 4.3 V (vs. Li), and charged ata constant voltage maintained at 4.3 V until the current reached 0.05 C.That is, each pouch cell was charged up to 100% of state of charge(SOC), that is, up to full charge capacity (FCC).

The charged lithium battery was left in an oven at 170° C. for 1 hourand exposed to heat, and the a battery voltage was measured.

In the lithium batteries prepared in Examples 1 and 2, the voltage ofthe lithium battery after the heat exposure is 90% or more of thevoltage of the lithium battery before the heat exposure, so that avoltage drop was insignificant.

In contrast, in the lithium battery prepared in Comparative Example 1,the voltage of the lithium battery after the heat exposure is 80% orless of the voltage of the lithium battery before the heat exposure, sothat a voltage drop was remarkable.

Therefore, in the lithium batteries of Examples 1 and 2, the voltagedrop of the lithium battery caused by side reactions due to heatexposure was suppressed, as compared with the lithium battery ofComparative Example 1. As a result, the reduction in output power of thelithium battery was suppressed.

Evaluation Example 5: Measurement of Whether Carboxyl Group was Present

The presence or absence of a carboxyl group was evaluated by measuringan IR spectrum of the cellulose nanofibers prepared in PreparationExample 1.

In the cellulose nanofibers of Preparation Example 1, a peak appeared inthe vicinity of 1,572 cm⁻¹ corresponding to a carboxyl group, from whichit was found that a carboxyl group was included therein.

Evaluation Example 6: Measurement of Content of Carboxyl Group

The content of a carboxyl group in the cellulose nanofibers ofPreparation Example 1 was measured, and the results thereof are shown inTable 1 below. The content of the carboxyl group may be measured by anyone of electrical conductivity titration and ion chromatography, but acombination of these methods was used herein to increase accuracy.

1. Electrical Conductivity Titration

The content of the carboxyl group was measured by electricalconductivity titration (Metrohm). 0.05 g of the cellulose nanofibers(CNFs) of Preparation Example 2 that were freeze-dried, 27 ml ofdistilled water, and 3 ml of 0.01 M NaCl were added to a 100 ml beaker,and the pH of the resulting solution was adjusted with 0.1 M HCl to pH 3or less. Subsequently, 0.2 ml of a 0.04 M NaOH solution was addeddropwise to the beaker until pH reached 10.5, and from a conductivityand pH curve, the carboxyl group content was calculated by Equation 1below. The measurement results thereof are shown in Table 1 below.

Carboxyl group content (mmol/g)=[0.04 M×volume of dropwise-added NaOH(mL)]/0.05 g  <Equation 1>

2. Ion Chromatography

5 ml of 12 mM HCl was added to 0.015 g of the cellulose nanofibers(CNFs) of Preparation Example 2 that were freeze-dried, followed bysonication for 1 hour. After maintaining the resulting solution at roomtemperature for 15 hours, the content of Na⁺ in the solution wasanalyzed by ion chromatography and the carboxyl group content wascalculated from the content of Na⁺ using Equation 2 below.

Carboxyl group content (mmol/g)=[mmol of Na⁺]/0.015 g  <Equation 2>

Evaluation Example 7: Measurement of Average Diameter of CelluloseNanofibers

The diameter of the cellulose nanofibers (CNFs) of Preparation Example 1was measured such that several images of an appropriately diluted CNFsolution were acquired using a transmission electron microscope (TEM)(manufactured by Titan Cubed, Super TEM), and were used to measure thediameter and length of 100 cellulose nanofibers using an image analyzer,and an average diameter and average length thereof were calculated. Inaddition, a full width at half maximum (FWHM) was calculated from adiameter distribution curve illustrating the number of cellulosenanofibers according to the diameter of 100 cellulose nanofibers. Themeasurement results thereof are shown in Table 1 below.

Evaluation Example 8: Measurement of Weight Average PolymerizationDegree of Cellulose Nanofibers

A degree of polymerization (DP) of the cellulose nanofibers (CNFs) ofPreparation Example 1 was calculated by a degree of polymerizationdetermined by viscosity measurement (DPv) and a weight average degree ofpolymerization (DPw).

5 mg of the freeze-dried CNFs was collected, and then 10 ml of pyridineand 1 ml of phenyl isocyanate were added into a 12 ml vial to induce aderivatization reaction at 100° C. for 48 hours. 2 ml of methanol wasadded to the sample, followed by washing twice with 100 ml of 70%methanol and washing twice with 50 ml of H₂O. Subsequently, themolecular weight, molecular weight distribution, and length distributionof the CNFs were measured by gel permeation chromatography (GPC). GPCwas performed using a Waters 2414 refractive index detector and a WatersAlliance e2695 separation module (Milford, Mass., USA) equipped withthree columns, i.e., Styragel HR2, HR4, and HMW7 columns. As an eluent,chloroform was used at a flow rate of 1.0 ml/min. The concentration ofthe sample was 1 mg/ml and an injection volume thereof was 20 μl.Polystyrene standards (PS) (#140) were used as references. Themeasurement results thereof are shown in Table 1 below.

TABLE 1 Average weight Carboxyl group Average degree of content diameterFWHM polymerization [mmol/g] [nm] [nm] [DPw] Preparation 0.11 18 23 5531Example 1

Referring to Table 1, it was found that carboxyl group-containingcellulose nanofibers were obtained.

As is apparent from the foregoing description, according to anembodiment, a side reaction of a separator with an electrolyte issuppressed at high temperature, thereby suppressing the deterioration ofpower of a lithium battery.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and “at least one” andsimilar referents in the context of describing the invention (especiallyin the context of the following claims) are to be construed to coverboth the singular and the plural, unless otherwise indicated herein orclearly contradicted by context. The use of the term “at least one”followed by a list of one or more items (for example, “at least one of Aand B”) is to be construed to mean one item selected from the listeditems (A or B) or any combination of two or more of the listed items (Aand B), unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

What is claimed is:
 1. A lithium battery comprising: a cathodecomprising a cathode active material; an anode comprising an anodeactive material; an electrolyte disposed between the cathode and theanode; and a separator impregnated with the electrolyte, wherein theseparator comprises cellulose nanofibers, and wherein a differentialscanning calorimetry (DSC) thermogram of the separator impregnated withthe electrolyte does not show an exothermic reaction peak as representedby a differential value (dH/dT) within a temperature range of about 150°C. to about 200° C.
 2. The lithium battery of claim 1, wherein theseparator impregnated with the electrolyte has a crystalline index afterexposure to heat at 170° C. for 3 hours under an inert atmosphere thatis 50% or more of the crystalline index of the separator before the heatexposure, wherein the crystalline index of the separator is expressed asan intensity ratio ((I₀₀₂−I_(AM))/I₀₀₂) of a crystalline peak intensity(I₀₀₂−I_(AM)) to a total peak intensity I₀₀₂ of a (002) crystal plane inan X-ray diffraction (XRD) spectrum, and the crystalline peak intensityis a difference between the total peak intensity I₀₀₂ of the (002)crystal plane and a peak intensity I_(AM) of an amorphous phase.
 3. Thelithium battery of claim 1, wherein, after the separator impregnatedwith the electrolyte is exposed to heat at 170° C. for 3 hours under aninert atmosphere, no carbonized areas are visible on a surface of theseparator.
 4. The lithium battery of claim 1, wherein the electrolytecomprises at least one selected from an ionic metal complex representedby Formula 1 and a lithium salt containing a sulfonyl group (—S(═O)₂—):

wherein, in Formula 1, Me is an element selected from transition metalsand elements belonging to groups 13 to 15 of the Periodic Table; M is ametal ion; a is an integer of 1 to 3, b is an integer of 1 to 3, and sis an integer equal to the value obtained by dividing b by a; p is 0 to8, q is 0 or 1, and r is 1 to 4; X₁ and X₂ are each independently O, S,or NR_(a); R₁ and R_(a) are each independently a halogen, a C₁-C₅ alkylgroup unsubstituted or substituted with one or more halogens, or a C₁-C₅aryl group unsubstituted or substituted with one or more halogens; andR₂ is a C₁-C₅ alkylene group unsubstituted or substituted with ahalogen, or a C₁-C₅ arylene group unsubstituted or substituted with oneor more halogens.
 5. The lithium battery of claim 4, wherein the ionicmetal complex is represented by Formula 2:

wherein, in Formula 2, Me is Al, B, or P; M is a metal ion; p is 0 to 8,q is 0 or 1, and r is 1 to 4; X₃ and X₄ are independently O or S; R₃ ishalogen; and R₄ is a C₁-C₅ alkylene group unsubstituted or substitutedwith one or more halogens, or a C₁-C₅ arylene group unsubstituted orsubstituted with one or more halogens.
 6. The lithium battery of claim4, wherein the ionic metal complex is represented by Formula 3 or 4:

wherein, in Formulae 3 and 4, Me is Al, B, or P; p is 0 to 8, and r is 1to 4; R₅ is a halogen; and R₆ is a C₁-C₅ alkylene group unsubstituted orsubstituted with one or more halogens.
 7. The lithium battery of claim4, wherein the ionic metal complex is represented by one of Formulae 5to 10:


8. The lithium battery of claim 4, wherein the lithium salt containingthe sulfonyl group is LiCF₃SO₃, LiC₄F₉SO₃, Li(CF₃SO₂)₂N, Li(C₂F₅SO₂)₂N,Li(FSO₂)₂N, Li(CF₃SO₃)₂N, or LiN(CxF_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂)wherein x and y are each independently an integer of 3 to
 10. 9. Thelithium battery of claim 4, wherein a content of at least one selectedfrom the ionic metal complex and the lithium salt containing thesulfonyl group is about 0.01 M to about 5 M based on a total weight ofthe electrolyte.
 10. The lithium battery of claim 1, wherein theelectrolyte comprises an organic solvent.
 11. The lithium battery ofclaim 10, wherein the organic solvent includes at least one selectedfrom the group comprising propylene carbonate, ethylene carbonate,fluoroethylene carbonate, butylene carbonate, dimethyl carbonate,diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate,ethyl propyl carbonate, methyl isopropyl carbonate, dipropyl carbonate,dibutyl carbonate, benzonitrile, acetonitrile, tetrahydrofuran,2-methyltetrahydrofuran, γ-butyrolactone, dioxolane, 4-methyldioxolane,N,N-dimethylformamide, dimethylacetamide, dimethylsulfoxide, dioxane,1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene,nitrobenzene, diethylene glycol, and dimethyl ether.
 12. The lithiumbattery of claim 1, wherein the cellulose nanofibers comprise carboxylgroup-containing cellulose nanofibers.
 13. The lithium battery of claim12, wherein a content of a carboxyl group in the carboxylgroup-containing cellulose nanofibers is 0.06 mmol/g or more.
 14. Thelithium battery of claim 12, wherein the carboxyl group-containingcellulose nanofibers comprise microbial cellulose nanofibers.
 15. Thelithium battery of claim 1, wherein the separator comprises: asingle-layered structure comprising a first layer comprising thecellulose nanofibers; a multi-layered structure comprising: a firstlayer comprising the cellulose nanofibers; and a second layer disposedon one surface of the first layer and comprising polyolefin; amulti-layered structure comprising: a first layer comprising thecellulose nanofibers; a second layer disposed on one surface of thefirst layer and comprising polyolefin; and a thirds layer disposed onanother surface of the first layer and comprising a polyolefin; amulti-layered structure comprising: a first layer comprising thecellulose nanofibers; and a second layer disposed on one surface of thefirst layer and comprising the cellulose nanofibers, and having acomposition different from that of the first layer; a multi-layeredstructure comprising: a first layer comprising the cellulose nanofibers;a second layer disposed on one surface of the first layer and comprisingthe cellulose nanofibers and having a composition different from that ofthe first layer; and a third layer disposed on another surface of thefirst layer and comprising the cellulose nanofibers, and having acomposition different from that of the first layer; a multi-layeredstructure comprising: a first layer comprising the cellulose nanofibers;a second layer disposed on one surface of the first layer and comprisingpolyolefin; and a third layer disposed on another surface of the firstlayer and comprising the cellulose nanofibers, and having a compositiondifferent from that of the second layer; a multi-layered structurecomprising: a first layer comprising polyolefin; a second layer disposedon one surface of the first layer and comprising the cellulosenanofibers; and a third layer disposed on another surface of the firstlayer and comprising the cellulose nanofibers; or a multi-layeredstructure comprising: a first layer comprising polyolefin; a secondlayer disposed on one surface of the first layer and comprising thecellulose nanofibers; and a third layer disposed on another surface ofthe first layer and comprising the cellulose nanofibers and having acomposition different from that of the second layer.
 16. The lithiumbattery of claim 1, wherein the cathode active material comprises alithium transition metal oxide having a nickel-containing layeredstructure, and wherein a content of nickel in the lithium transitionmetal oxide is 60 mol % or more with respect to a total number of molesof transition metals.
 17. The lithium battery of claim 16, wherein thelithium transition metal oxide is represented by Formula 11:Li_(a)Ni_(x)CO_(y)M_(z)O_(2−b)A_(b)  <Formula 11> wherein, in Formula11, 1.0≤a≤1.2, 0≤b≤0.2, 0.6≤x<1, 0<y≤0.3, 0<z≤0.3, and x+y+z=1 aresatisfied, and M is at least one selected from manganese (Mn), vanadium(V), magnesium (Mg), gallium (Ga), silicon (Si), tungsten (W),molybdenum (Mo), iron (Fe), chromium (Cr) Zinc (Zn), titanium (Ti),aluminum (Al), and boron (B), and A is F, S, Cl, Br, or a combinationthereof.
 18. The lithium battery of claim 16, wherein the lithiumtransition metal oxide is represented by one of Formulae 12 and 13:LiNi_(x)CO_(y)Mn_(z)O₂  <Formula 12>LiNi_(x)CO_(y)Al_(z)O₂  <Formula 13> wherein, in Formulae 12 and 13,0.6≤x≤0.95, 0<y≤0.2, 0<z≤0.1, and x+y+z=1 are satisfied.
 19. The lithiumbattery of claim 1, wherein the anode active material comprises at leastone selected from a silicon-based compound, a carbon-based compound, acomposite of a silicon-based compound and a carbon-based compound, and asilicon oxide (SiO_(x), 0<x<2).
 20. The lithium battery of claim 1,wherein, after the lithium battery is charged up to a state of charge of100% and then exposed to heat at 170° C. for 1 hour, a voltage of thelithium battery after the heat exposure is about 90% or more of avoltage of the lithium battery before the heat exposure.